There have been a number of developments recently to improve the characteristics of heat-sensitive recording papers. These developments have been applied in practice to output paper for facsimiles and computers.
Many of these heat-sensitive recording papers are called the dye type. Examples of dye type papers are disclosed in U.S. Pat. Nos. 3,451,338 and 3,539,375, Japanese Patent Publication No. 4160/68 (U.S. patent application Ser. No. 512,546), Japanese Patent Application (OPI) No. 27253/80 (The term "OPI" as used herein refers to a "published unexamined Japanese patent application"), and so on.
In general, it is advantageous to use heat-sensitive coloring papers as recording papers because the recording apparatus therefor can be light weight and miniaturized. In recent years this advantage has rapidly improved the chances of using these heat-sensitive recording papers. However, a use of these heat-sensitive recording papers is not desirable in that recording cannot be carried out at a sufficiently high speed. In order to carry out high speed recording, a large amount of heat energy must be applied on the heat-sensitive recording paper in a short time because the amount of heat energy per unit area necessary for recording is constant. However, the recording element has a limited recording energy depending on the power of source. In order to subjugate this defect, various means have been devised both with respect to the recording apparatus and recording paper. A typical example thereof is the improvement of the surface smoothness of the heat-sensitive recording paper involving calendering processing.
However, such a surface processing cannot make the recording paper smooth without various disadvantages. One disadvantage is the occurrence of fog, that is, the color reaction which takes place in the course of the surface treatment resulting in undesired coloration in the recording paper. A proposed means of eliminating the disadvantage is the addition of grain-shaped wax. This was proposed in Japanese Patent Publication No. 14531/75 and U.S. Pat. Nos. 4,032,690 and 3,445,261. However, waxes usually have large heat capacities and require great heat upon fusion. Consequently, they retard the thermal response of heat-sensitive recording papers.
Another disadvantage is a lowered facility with respect to writting with a pencil, a ball-point pen or the like.
Yet another disadvantage is impediment to running of a recording paper on the recording element. More specifically, the recording paper sticks to the recording element at colored parts because the recording element is brought into very close contact with the recording paper.
Notwithstanding the disadvantages as described above, surface treatment to improve smoothness is carried out because the comparatively large contribution of improved smoothness gives toward increasing recording speed.